There has heretofore been used a two-piece can having a can wall and a can bottom molded integrally together without a junction portion in the can wall as a metal container for containing foods, beverages and aero sols. The two-piece can is formed through severe workings such as draw-ironing, draw working followed by stretch working, or draw working followed by stretch working and, further, ironing working (stretch-ironing).
The two-piece can has been produced by using a resin-coated steel plate obtained by coating a steel plate with a resin. From the standpoint of adhesion between the steel blank and the resin coating, work adhesion properties and corrosion resistance after the working, in particular, an electrolytic chromium/chromium oxide-coated steel plate (tin-free steel, hereinafter referred to as TFS) has been extensively used for producing cans having a reduced thickness through a severe working in which a ratio represented by the following formula (1) and an average plate thickness reduction ratio represented by the following formula (2) are,Height (H) of can/can diameter (D)≧1  (1){(Initial plate thickness (t0)−can wall thickness (t))/initial plate thickness (t0)}×100≧20%  (2)
Japanese Unexamined Patent Publication (Kokai) No. 11-140691 teaches that a thermoplastic resin-coated steel plate obtained by forming a silane-treated film and a thermoplastic resin film on the TFS is suited for the use that is to be subjected to a severe working.
The resin-coated steel plate using the above TFS features excellent adhesion of the coated layer even when subjected to the above severe working and exhibits excellent corrosion resistance after the working. However, the TFS is the one obtained through a cathodic electrolytic treatment in a treating solution containing hexavalent chromium followed by the washing with water. Though hexavalent chromium is not contained in the TFS surface-treating film which is the finally formed product, the treating solution contains harmful hexavalent chromium arousing many problems from the environmental point of view.
That is, when the TFS is to be used, the drain water and exhaust gases of the treating solution containing hexavalent chromium must be completely treated so will not to be drained to the external side. Therefore, a tremendous amount of cost is required for the construction of the drain water treatment facility, exhaust gas treatment facility, and disposal of the waste. Besides, transporting the sludge of waste water and disposal thereof encounter stringent regulations. It has therefore been desired to produce thin cans through the above-mentioned severe working but by using a resin-coated steel plate obtained by using a metal blank other than the TFS.
A tin plate (tin-plated steel plate) has long been used. Usually, however, the tin plate is obtained by plating tin followed by the chemical treatment such as being immersed in a solution of dichromic acid or the electrolysis in this solution. It is further widely effected to coat the tin plate with a resin film in advance without, however, capable of withstanding the severe working for accomplishing a ratio of Height (H) of can/can diameter (D)≧1 and a plate thickness reduction ratio of not smaller than 20%.
In the press-formed can using the tin plate, therefore, a protection film must be formed by applying a coating material after the press-molding, arousing the problems of disposal and treatment of the organic solvent, and coping with the worsening working environment. It has further been desired to shorten the step of coating.